1. Field of the Invention
The present invention generally relates to location systems. More particularly, the present invention relates to determining a location of a receiver.
2. Related Art
There are several conventional location systems. Each conventional location system has particular advantages and disadvantages with respect to indoor applications. These conventional location systems enable the determination of the location of a receiver, whereas the receiver may be any type of electronic device.
A first conventional location system utilizes one or more base stations to determine the location of a receiver. A first base station transmits a signal. The receiver receives the signal and retransmits the signal to the first base station. The first base station calculates the phase angle difference between the original transmitted signal and the retransmitted signal to determine the distance between the first base station and the receiver. A similar process is performed between the receiver and other base stations to determine the location of the receiver.
Unfortunately, in indoor applications the first conventional location system may inaccurately determine the location of the receiver because of the multipath reflections problem. In particular, the base station may receive a first retransmitted signal which reflected off several surfaces before arriving at the base station (long path case), may receive a second retransmitted signal which reflected off a few surfaces before arriving at the base station (medium path case), and may receive a third retransmitted signal which reflected off no surfaces before arriving at the base station (short path case). Thus, the base station may inaccurately calculate the distance between the base station and the receiver depending on which retransmitted signal (i.e., long path case, medium path case, or short path case) is received.
A second conventional location system utilizes beacon transmitters which use infrared (IR) technology to communicate with the receiver and determine the location of the receiver. However, the receiver must be close to the IR beacon transmitter. Moreover, the transmit IR cone and the receive IR cone between the receiver and IR beacon transmitter must intersect. Furthermore, large scale distribution of the IR beacon transmitters is expensive (since IR communication is feasible only over a short distance) and requires extensive maintenance for the large number of IR beacon transmitters needed for the large scale distribution.
In addition, a third conventional location system uses GPS signals to determine the location of the receiver. Unfortunately, the GPS signals are too weak and inaccurate to provide a reliable determination of the location of the receiver in indoor applications.
Furthermore, a fourth conventional location system uses time of flight techniques to determine the location of the receiver. A time of flight location system using radio frequency (RF) exists but requires tricky timing correlation. Moreover, with standard modulation schemes, multipath interference may introduce significant errors. An acoustic time of flight location system is much simpler. However, it is of limited use in indoor applications due to sound obstruction. Safety concerns limit the power and thus the range of acoustic systems. Present implementations involve stations placed on a four foot grid, representing a tremendous installation and maintenance cost.
Moreover, a fifth conventional location system utilizes a signal fading scheme implemented using existing radio channels or using yet approved radio channels to determine the location of the receiver. This location system has low accuracy and in many cases needs to be trained.
A location system is described. The location system includes a plurality of beacon transmitters each positioned at a respective location. Each beacon transmitter includes a plurality of antennas positioned in a circular arrangement, wherein each beacon transmitter is configured to transmit an identification signal having a plurality of reference data and to transmit a directional signal from the plurality of antennas by selecting one of the antennas at a time in sequence around the circular arrangement to simulate a rotating antenna.
Moreover, the location system further includes a receiver configured to receive the plurality of identification signals and a plurality of Doppler-shifted directional signals each corresponding to one of the directional signals, wherein the receiver is configured to generate a plurality of time data for each received Doppler-shifted directional signal, and wherein the receiver is configured to determine a location of the receiver using each received Doppler-shifted directional signal, each time data, and each identification signal.